where is the summoning spot for Placidusax?

[Gearwar]

Im trying to help people to fight that thing but I am not sure where to place my sign.

You can not do it in the arena where you fight him. I tried doing it next to the "lie down" prompt area and on the cliffs above.

Nothing. Im only SL lvl 120 and so far I got instant summoned to pretty much any boss encounter. Including The very late bosses.

[Sparhawk 59]

its where you lay down